1. Field of the Invention
This invention relates to a method for operating a multi-stage hydraulic machine, and more particularly to a method for stopping a multi-stage hydraulic machine having an inlet valve, plural guide vanes including highest and lowest stage guide vanes, and a return passage connected between flow passages of respective stages.
The term "multi-stage hydraulic machine" as used in this specification and in the appended claims includes a multi-stage water-turbine and a multi-stage pump-turbine which can be operated as a turbine.
2. Description of the Prior Art
In general, the operation of a multi-stage water-turbine or a multi-stage pump-turbine is performed by controlling openings of guide vanes arranged circumferentially around the runners so as to adjust the flow rate of water that passes through the runners.
Hereinafter, description will be made with respect to a Francis type two-stage pump-turbine as an example of a multi-stage hydraulic machine. FIG. 1 is schematic diagram showing a Francis type two-stage pump-turbine, wherein reference numeral 1 designates a runner of a low stage, reference numeral 2 designates movable guide vanes of the low stage, reference numeral 3 designates a runner of a high stage, and reference numeral 4 designates movable guide vanes of the high stage, respectively. The high stage portion communicates with the low stage portion through a return passage 5. Reference numeral 6 designates a casing, reference numeral 7 designates an inlet valve, reference numeral 8 designates a penstock and reference numeral 9 designates a draft tube.
When the two-stage pump-turbine with such a configuration operates as a turbine, water that flows from the penstock 8 through the inlet valve 7 into the casing 6 is drained to a trailrace (not shown) through a route of the movable guide vanes 4, the runner 3, the return passage 5, the movable guide vanes 2, the runner 1 and the draft tube 9. On the other hand, when the two-stage pump-turbine operates as a pump, the runner 1 pumps water up to an upper dam (not shown) through a route opposite to that in case of the turbine operation.
When the two-stage pump-turbine is operating as a turbine, it is necessary to control the movable guide vanes to close rapidly in case of stoppage from normal turbine operation or from turbine load interruption upon occurrence of failure. Closure of the movable guide vanes causes the water flow rate to be varied, so that water pressure fluctuation will be induced due to a water hammering phenomenon resulting from the variation in the water flow rate in the movable vanes. This produces water pressure waves which are propagated into the respective flow passages. The water pressure waves are reflected mainly at the narrowest portion of the flow passages and then propagated to the flow passages outside the narrowest portion. Thus, when the water pressure fluctuation is abnormally large, the flow passages are in danger of partial damage or even a severe breakdown. In the two-stage pump-turbine, the return passage 5 is shaped of such a complicated flow passage configuration that the structural strength of the return passage is liable to be weakened, which causes a serious problem. Therefore, a safe and assured control method for the two-stage pump-turbine is needed such that the water pressure waves are not directly applied to the return passage 5.
Hereinafter a conventional method for stopping the two-stage pump-turbine operating as a turbine will be described. In case of stopping the two-stage pump-turbine, firstly the movable guide vanes 4 and 2 are controlled to be closed in concert, or they are controlled to be closed in order with a predetermined time period between first closing of the movable guide vanes 4 and subsequent closing of the movable guide vanes 2. Next, the inlet valve 7 is controlled to be closed after the movable guide vanes 4 are completely closed. Water pressure fluctation in this case is applied to the outer portions of the movable guide vanes 4 and 2, in which the flow passages thereof are narrowed, namely to the flow passage of the casing 6 and the return passage 5, respectively. The water pressure within the flow passage of the casing 6 varies in accordance with the opening of the movable guide vanes 4, while the water pressure within the return passage 5 varies in accordance with the opening of the movable guide vanes 2. Accordingly, the water pressure waves due to the water pressure fluctation in the penstock 8 derived from the water hammering phenomenon are mainly applied to the flow passage of the casing 6, and not directly to the return passage 5. Under normal operation, the return passage 5 only receives a water pressure component which is assigned to the runner 1.
Next, description will be made as to the case when a failure occurs during control of closing of the movable guide vanes 4 and 2. After commencement of the stopping control of the movable guide vanes 4 and 2, wherein vanes 4 and 2 are controlled to be closed in concert or in order, if a failure occurs during closing such that the closing speed of the movable guide vanes 4 becomes slower than a predetermined speed, or the closing speed of the movable guide vanes 2 becomes faster than another predetermined speed, the opening of the movable guide vanes 2 may become equal to or smaller than that of the movable guide vanes 4. Under such a condition, the water pressure wave from the penstock 8 will be applied to the flow passage outside the movable guide vanes 2, i.e., the return passage 5, instead of the flow passage of the casing 6 to which the water pressure wav is to be applied during normal closing control operation. The resultant water pressure fluctuation in the return passage 5 continues to be abnormally large until the movable guide vanes 2 are completely closed, and this may cause either partial damage or a severe breakdown in the return passage 5, which is a serious problem.
As described above, there is a need for a method for stopping a multi-stage hydraulic machine operating as a turbine, which method can stop the multi-stage hydraulic machine without applying water pressure waves arising due to the water pressure fluctuation to the return passage even when a failure occurs in the closing control of the movable guide vanes.